In recent years, it has been seriously desired to reduce carbon dioxide emission in order to cope with air pollution and global warming. In the automobile industry, there are increasing hopes that introduction of electric vehicles (EV), hybrid electric vehicles (HEV) and the like will reduce carbon dioxide emission. For this reason, development of key electric devices to practical use of these vehicles, such as secondary cells for driving a motor, has been actively made.
Among secondary cells for driving a motor, lithium-ion secondary cells have drawn attention for their high theoretical energy, and the development thereof has been being made in a rapid pace. In general, a lithium ion secondary cell includes a cathode formed by applying a cathode slurry containing a cathode active material on the surface of a current collector, an anode formed by applying an anode slurry containing an anode active material on an anode current collector, an electrolyte disposed between the cathode and the anode, and a cell case that houses the cathode, the anode and the electrolyte.
A type of such lithium ion secondary cells for driving a motor is non-aqueous organic electrolytic solution secondary cells that use a combination of a spinel lithium manganate (e.g. LiMn2O4), a layered lithium nickel oxide (e.g. LiNi1-xCOxO2) and the like for the cathode active material, a carbon/graphite-based material for the anode active material, and a non-aqueous organic electrolytic solution (LiPF6/EC/DEC) for the electrolytic solution (for example, see Japanese Patent Unexamined Publication No. Hei9-55211).
A problem with such layered lithium-containing transition metal oxides is that their material properties and cell properties greatly depend on not only their composition but also their synthesis method and synthesis conditions. Further, non-aqueous electrolytic solution secondary cells that use a layered lithium-containing transition metal oxide for the cathode active material suffer from insufficient performance (charge-discharge capacity, rate characteristics), insufficient cycle life (capacity retention), low repeatability of the cell properties, wide variation between production lots and the like. On the other hand, cells for automobiles require not only high capacity per unit mass of the active material but also a large storable amount of electricity per unit capacity of the cells.
To cope with the above-described problems and the demands, conventional lithium-ion secondary cells using a layered lithium-containing transition metal oxide (solid solution cathode material) have been subjected to an electrochemical treatment (activating treatment) for increasing the capacity of the cathode active material in which a charge-discharge treatment within or over the potential plateau region of the cathode active material is carried out at least once by applying a voltage of 4.3 V to 4.75 V (for example, see Japanese Patent Unexamined Publication No. 2011-66000).